refactor(backend): phase 4 - add test hooks and extend service layer

- Extract internal functions in commands/mcp.rs and commands/provider.rs
  to enable unit testing without Tauri context
- Add test hooks: set_mcp_enabled_test_hook, import_mcp_from_claude_test_hook,
  import_mcp_from_codex_test_hook, import_default_config_test_hook
- Migrate error types from String to AppError for precise error matching in tests
- Extend ProviderService with delete() method to unify Codex/Claude cleanup logic
- Add comprehensive test coverage:
  - tests/mcp_commands.rs: command-level tests for MCP operations
  - tests/provider_service.rs: service-level tests for switch/delete operations
- Run cargo fmt to fix formatting issues (EOF newlines)
- Update BACKEND_REFACTOR_PLAN.md to mark phase 3 complete

src-tauri/src/app_config.rs

@@ -90,8 +90,8 @@ impl MultiAppConfig {
         }
 
         // 尝试读取文件
-        let content = std::fs::read_to_string(&config_path)
-            .map_err(|e| AppError::io(&config_path, e))?;
+        let content =
+            std::fs::read_to_string(&config_path).map_err(|e| AppError::io(&config_path, e))?;
 
         // 检查是否是旧版本格式（v1）
         if let Ok(v1_config) = serde_json::from_str::<ProviderManager>(&content) {